Mn spin dynamics is measured in very diluted (Cd,Mn)Te crystals by time-resolved Kerr rotation. Spin beats due to the hyperfine interaction between the 3d electrons of the Mn ions and their own nuclei are detected. It is shown that the effect of the crystal field can be strongly suppressed for "magic" orientations of the magnetic field. This particular orientation of the field permits the optical read-out of the Mn nuclear spin state. Manganese ions trapped on a semiconductor lattice have uniform properties and relatively long spin lifetimes, which make them promising for optical manipulation. In particular, Mn2+ ions embedded in a II-VI semiconductor are S-state ions, weakly coupled to the lattice. For this reason quite long Mn electronic spin relaxation times are expected and have been observed [1]. Here we show that the spin coherence time is mainly limited by dipole-dipole interactions at a Mn concentration x=0.001, and reaches up to 15 ns. At this low concentration, fine and hyperfine structures of Mn2+ are resolved in electron spin resonance experiments. In the time-domain it corresponds to low-frequency beats as shown in Fig. 1.

Optical pump-probe experiments reveal spin beats of manganese ions in (Cd,Mn)Te, due to hyperfine and crystal fields. At "magic" orientations of the magnetic field, the effect of local crystal field is strongly suppressed. In this case, the spin precession of Mn2+ embedded in the lattice approaches the precession expected for the free ion. Following optical excitation, regular spin pulses show up, revealing the one-to-one correspondence between precession frequency and Mn2+ nuclear spin state. The period of the spin pulses accurately determines the hyperfine constant |A|=705 neV. The manganese spin coherence time up to T2Mn≃15 ns is measured for a manganese concentration x=0.0011.

Terahertz light helicity sensitive photoresponse in GaAs/AlGaAs high electron mobility transistors. The helicity dependent detection mechanism is interpreted as an interference of plasma oscillations in the channel of the field-effect-transistors (generalized Dyakonov-Shur model). The observed helicity dependent photoresponse is by several orders of magnitude higher than any earlier reported one. Also, linear polarization sensitive photoresponse was registered by the same transistors. The results provide the basis for a new sensitive, all-electric, room-temperature, and fast (better than 1 ns) characterisation of all polarization parameters (Stokes parameters) of terahertz radiation. It paves the way towards terahertz ellipsometry and polarization sensitive imaging based on plasma effects in field-effect-transistors.

AlGaN/GaN based field effect transistors for terahertz detection and imaging

AlGaN/GaN based FETs have great potential as sensitive and fast operating detectors because of their material advantages such as high breakdown voltage, high electron mobility, and high saturation velocity. These advantages could be exploited for resonant and non-resonant terahertz detection. We have designed, fabricated, and characterized AlGaN/GaN based FETs as single pixel terahertz detectors. This work focuses on non-resonant detection and imaging using GaN field plate FETs. To evaluate their performances as terahertz detectors, we have measured the responsivity as a function of gate voltage, the azimuthal angle between the terahertz electric field, the source-to-drain direction, and the temperature. A simple analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics and it fully describes the experimental results on the non-resonant sub-terahertz detection by the AlGaN/GaN based FETs. The imaging performances are evaluated by scanning objects in transmission mode and an example of application of terahertz imaging as new non-destructive technique for the quality control of materials is given. Results indicate that these FETs can be considered as promising devices for terahertz detection and imaging applications.

We study the broadband photovoltaic response of field effect transistors on terahertz radiation. A simple physical analytical model of the response is developed. It is based on plasma density perturbation in the transistor channel by the incoming terahertz radiation. The model shows how the non-resonant detection signal is related to static (dc) transistor characteristics. We analyze loading effects related to capacitive, inductive, and resistive coupling of the detector to the read-out circuit as a function of modulation frequencies and loading resistors. As we show, the proposed physical model completed by loading effects fully describes the experimental results on the non-resonant sub-terahertz detection by all studied III-V (GaAs, GaN) and silicon based transistors. Field effect transistors were recently proposed as the best terahertz detecting pixels for fabrication of low cost focal plane arrays for terahertz imaging. This article gives prospects for electrical simulation of these transistors and their optimal integration in the focal plane arrays.